Currently, a critical dimension scanning electron microscope (hereinafter, abbreviated as CDSEM) is mainly used to measure a dimension of a semiconductor device pattern. The structure of the CDSEM is basically the same as a scanning electron microscope. First of all, an electron discharged from a thermal or field emission electron source is accelerated. Thereafter, an electron beam obtained by reducing a diameter of the electron beam using a lens is formed. By two dimensionally scanning the electron beam on the sample (for example, a semiconductor wafer or a reticle) and detecting a generated secondary electron or a reflection electron, it is possible to acquire a minute pattern scanning image formed on the sample.
Some new materials that are recently introduced to a process have weak resistance against the electron beam. Therefore, it is required to reduce the damage caused by lowering energy of an incident electron. However, as the energy of the electron beam becomes lower, the resolution becomes deteriorated. Therefore, the lowering of energy contradicts a trend of the high resolution of the CDSEM required for the minuteness of a circuit pattern. Therefore, an accelerating voltage after electrons are generated from the electron source is set to be high, and then a mechanism that applies a decelerating electric field (that is, applies retarding voltage) before the electrons enter onto the sample is provided. Therefore, it is possible to achieve both increasing of resolution and lowering of damage of an image to be acquired.
In the meantime, some samples to be measured may have a surface formed of an insulating material. A trajectory of an electron beam after reducing the speed is bent due to the charging or distribution of the surface of the sample. Accordingly, it is proved that the focus position displacement or astigmatism of the CDSEM occurs. There are two reasons of charging of the sample: one is that the sample is originally charged, and the other is that the sample is charged by irradiating the electron beam by the CDSEM. As a reason that the sample to be carried in is charged, the polarization of organic materials that forms a resist, caused by the friction at the time of resist applying process and the fixation thereof are estimated. However, all reasons of charging cannot be explained. It is considered that the charging is caused by fixed charges that remain even when the sample is grounded.
Examples of charging caused by irradiating an electron beam include charging occurring by an incident primary electron at the time of observing sample observation or acquiring an image for measuring an automatic recipe. If the above-mentioned charging occurs, a conversing position of the incident electron beam is displaced from the surface of the sample by the change in the trajectory of the charged particle incident onto the sample, such that focus position displacement or astigmatism occurs. Therefore, it is required to adjust the focus position displacement or to correct the astigmatic difference, which results in lowering the throughput. Further, if the charging has in-plane distribution and the charging amount is varied between the chips on the sample, whenever the measuring chip is changed, the focusing adjustment or astigmatism correction described above is required, which results in lowering the throughput.
In this regard, Patent Literature 1 discloses a technology of measuring the astigmatic difference from the distance between the focuses in which a contrast of a differential image becomes maximum in perpendicular directions in different focal positions by image processing. Further, Patent Literature 2 discloses a technology that one part of an on-axis beam is blocked by an aperture and the astigmatism is corrected so that the amount of movement of the beam on the sample becomes minimum when the focal position is deviated, in consideration of a parallax method.
The above-mentioned scanning electron microscope accelerates a primary electron discharged from an electron source and converses the primary electron using an electrostatic or electromagnetic lens to irradiate the primary electron onto a surface of a sample. A secondary electron is generated from the sample by irradiating the primary electron. If the irradiated electron beam is scanned on the surface of the sample to acquire an intensity of the secondary electron signal, the amount of secondary electrons generated from edge parts of various patterns formed on the surface of the sample is increased. Therefore, an electron microscope image (SEM image) onto which the shape of the sample is reflected is acquired. A bright portion corresponding to the edge part having the shape of the sample represented in the image is referred to as a white band. Further, scanning electron microscopes are disclosed, for example, in Patent Literatures 3 to 6.